1. Field of the Invention
This invention relates generally to a platinized tin oxide-based catalyst. It relates particularly to an improved platinized tin oxide-based catalyst able to decompose nitrogen oxide species (NOx) to nitrogen and oxygen without the necessity of a reducing gas.
2. Description of the Related Art
Nitrogen oxides (NOx), including NO, are generated by high-temperature combustion processes, such as those that occur in internal combustion engines. Nitrogen oxides are corrosive and contribute to acid rain; therefore, their presence in the atmosphere is undesirable. Fortunately, nitrogen oxides are thermodynamically unstable at low and near ambient temperatures and should decompose at such temperatures to nitrogen and oxygen. Unless accelerated by catalysis, however, this decomposition occurs so slowly as to be inconsequential during practical time spans. As a result, considerable effort has gone into the development of a catalyst to accelerate this decomposition.
Many catalysts have been developed that will decompose nitrogen oxides in reducing environments. Heretofore, however, no catalyst has been developed that will decompose nitrogen oxides in non-reducing environments. This results in many disadvantages, including reduction of the efficiency of internal combustion engine systems.
Automotive catalytic converter technology has changed little since its inception over 25 years ago when automotive emission regulations were first implemented. Typical catalyst coatings consist of a series of aluminum oxide (alumina) washcoat- and precious-metal layers baked on the honeycomb channels of a ceramic substrate. The thick (˜150 microns) catalyst coating comprises approximately 30% of the total weight of the substrate. These coated “bricks” are then assembled and sealed inside a stainless steel can to allow coupling to the automotive exhaust manifold. As EPA emission regulations have tightened, the industry response has been to increase the size of the bricks, increase precious metal loading, and move the catalytic converter in closer proximity to the engine, thereby increasing exhaust temperatures for improvement in catalytic activity. The outcome of these changes has been ever increasing costs for catalytic converter products. In addition, these changes have had a negative impact on automobile fuel efficiency.
In response to the need for the next generation of catalysts for automotive applications, low-temperature oxidation catalysts were developed by NASA Langley Research Center. These improved catalysts are described in U.S. Pat. Nos. 4,829,035; 4,839,330; 4,855,274; 4,912,082, 4,991,181, 5,585,083; 5,948,965; 6,132,694; 7,390,768; and 7,318,915 which patents are hereby incorporated by reference herein as if set forth in their entireties. These catalysts exhibit several key advantages over the current state-of-the-art. First, unlike the thick, inert layer of alumina used in conventional catalyst technology, generally these catalysts can use a single active tin oxide-based coating (<5 microns) that enhances the catalytic performance by acting as an oxygen storage device. Second, their active washcoat reduces the temperature (i.e., light off) at which the catalyst begins converting toxic to nontoxic gases, as well as, requiring less precious metal to attain the same toxic gas conversion efficiency over time. Third, these catalysts are capable of capturing enough oxygen from the natural exhaust stream to effect the chemical reactions. Unlike traditional catalytic converter technology, external air sources and ancillary sensors, air pumps, and hoses are not required for catalytic converter operation.
Despite their improvement over existing catalysts, these low-temperature tin-oxide catalysts failed to decompose nitric oxide to nitrogen and oxygen without the necessity of a reducing gas. There still exists a need for such a catalyst.